Microlithographic projection exposure methods are used for fabricating semiconductor components and other finely structured components. Use is made of masks (reticles) that bear the pattern of a structure to be imaged, for example a line pattern of a layer of a semiconductor component, such as an integrated circuit (IC). A mask is positioned in a projection exposure apparatus between an illumination system and projection objective in the region of the object surface of the projection objective, and illuminated with illumination radiation provided by the illumination system. The radiation varied by the mask and the pattern forms projection radiation propagating through the projection objective, which images the pattern of the mask onto the substrate to be exposed, which normally bears a radiation-sensitive layer (photoresist).
A certain fraction of energy of radiation used for the exposure process is absorbed by optical elements within the projection objective, the amount of absorption typically being dependent on the material subject to radiation energy. Energy absorption typically causes heating of the optical elements subject to the radiation. The heating of lens groups and other optical elements caused by radiation energy will be referred to as “lens heating” in the following. Lens heating may cause surface deformations of optical elements and/or changes of refractive index of transparent materials directly through increasing the temperature and/or indirectly by mechanical strain caused by heating. Both refractive index changes and surface deformations of optical surfaces cause changes of the optical properties of single optical elements as well as of optical systems including a plurality of optical elements.
Lens heating is typically not considered in standard processes for designing optical systems. Therefore, typical optical designs are optimized for a “cold condition” not accounting for lens heating effects. However, lens heating may cause deterioration of optical properties in optical systems optimized for “cold condition” during operation. Optical performance may still be acceptable where operation conditions are met which do not cause significant lens heating effects. Lens heating effects occurring during operation of a projection exposure apparatus may be compensated for by providing appropriate manipulation devices capable of dynamically influencing the imaging properties of a projection objective, for example by actively moving and/or deforming selected optical elements.
While a compensation of lens heating effects through external manipulation devices may be possible where the lens heating is essentially uniform or rotationally symmetric on important optical elements, compensation may be difficult where non-uniform lens heating effect occurs on important optical elements. For example, where a projection objective is designed to image an off-axis object field onto an off-axis image field, the radiation load on optical elements close to field surfaces (i.e. object surface, image surface, optional intermediate images) may be non-rotationally symmetric. Further, state-of-the-art projection exposure methods often use off-axis illumination, which enables smaller features to be faithfully imaged. With this technique a mask providing the pattern is illuminated at oblique (non-perpendicular) angles, which may improve resolution, but particularly improves the process latitude by increasing depth of focus (DOF) and/or contrast. Off-axis illumination modes include multipole illumination in which an intensity distribution at a pupil plane of the illumination system acting as an effective source is characterized by several poles (spatially confined regions of high intensity) which are not on the optical axis (off-axis). The spatial intensity distribution at the pupil surface is converted into an angular distribution at the plane of the mask and further creates corresponding non-uniform irradiation loads particularly in optical elements of the projection objective arranged at or close to a pupil surface of the projection objective optically conjugated to the pupil plane of the illumination system where the effective source is generated.